Incorporating heterogeneous lacunary Keggin anions as efficient catalysts for solvent-free cyanosilylation of aldehydes and ketones

Polyoxometalates (POMs) as efficient catalysts can be used a wide range of chemical transformations due to their tunable Brønsted/Lewis-acidity and redox properties. Herein, we reported two hybrid and heterogeneous lacunary Keggin catalysts: (TBA)7[PW11O39] (TBA-PW11) and (TBA)8[SiW11O39]·4H2O (TBA-SiW11) (TBA+: tetrabutylammonium) in which [XW11O39]n− anions were coated by TBA+ cations. In this form, TBA+ can easily trap reactants on the surface of the catalysts and increase the catalytic reaction. Therefore, the catalytic performance of both POMs was tested in cyanosilylation of numerous compounds bearing-carbonyl group and trimethylsilyl cyanide under solvent-free conditions. TBA-PW11 is more effective than TBA-SiW11, conceivably due to the higher Lewis acidity of the P than the Si center and to the higher accessibility of the metal centers in the framework structure. Noteworthy, the recyclability and heterogeneity of both POMs catalysts were also examined, and the results confirmed that they remain active at least after three recycling procedures.

www.nature.com/scientificreports/ be prepared by removing one or more addenda atoms from the complete structure. The removal of one, two, or three addenda metals will respectively lead to the formation of mono-, di-, and tri-lacunary species. This operation is mainly controlled by the variation of the pH of the solution to tailor the desired structure. Lacunary Keggin possesses a higher negative charge than its complete form (anionic charge of the POM)/(number of nonhydrogen atoms of the POM) 22 . Up to now, Keggin-type POMs have been widely used as an oxidation catalyst 31 , and those containing P as the heteroatom showed higher catalytic activity. This behavior can be explained with different electronegativity of the heteroatoms, (P (2.19) > Si (1.90) > Al (1.61)). Fully, the lower electronegativity of the heteroatom leads the more polarized bond between this atom and the oxygen bridging atom as well as the addenda metal sites, resulting to an increase in the basicity of the POM 28,32,33 .
POMs are normally soluble in both water and polar organic solvents and counter-cations play an essential role in the solubility of POMs. For example, POMs with small cations such as Na + or H + are highly soluble in water and other polar organic solvents. On the other hand, POMs with large cations such as Cs + , tetrabutylammonium (TBA + ), or dimethyldioctadecylammonium (DODA + ) are insoluble in water and exhibit the low absorptive capacity for polar molecules. Therefore, the later groups can be categorized as heterogeneous catalysts 34,35 .

Results and discussion
Synthesis and characterization of catalysts. In this study, two heterogeneous nanocatalysts, TBA-PW 11 and TBA-SiW 11 , were obtained by top-down approach with ultrasonic method upon 15 min of sonication. According to the SEM images, morphologies of TBA-PW 11 and TBA-SiW 11 can consider as rhombic and cubic (Fig. 2). Furthermore, the presence of O, C, and W, in the nanocatalysts is confirmed by the EDS spectrum ( Fig. 2).
Also, there are several examples that the outer surface of anionic POMs can be surrounded by organic cations (like TBA + ) 45,46 and in the case of our catalysts, 1 H NMR and 13 C NMR spectra provide clear and direct evidence for the presence of TBA + . For example, in the 1 H NMR, three peaks located separately around 1.35, 1.60, and 3.20 ppm can be assigned to the CH 2 of TBA + and the CH 3 group located at 0.96 ppm (Fig. 3). Also, The 31 P NMR spectrum of TBA-PW 11 (Fig. S1) was in the normal range of diamagnetic phosphotungstate and showed one peak at − 13.21 ppm, corresponding to the P atoms in the lacunary anion 47 .
The IR spectra of POMs include characteristic metal-oxygen stretching vibrations that occur in the specific region (generally sharp bands between 400−1000 cm −1 ). The characteristic strong bands for X-O, W-O t , W-O b and W-O c stretching vibrations of nanocatalysts are shown in Table 1 and Fig. S2, which can approve the exact structures of the final catalysts. Moreover, absorptions in the range 2873-2961 cm -1 correspond to the C-H stretching vibrations of TBA + . Specific bands at around 1700 and 3300 cm -1 assigned to water molecules 48 . Catalytic activity. To extend the catalytic capacity of TBA-PW 11 and TBA-SiW 11 catalysts, herein, we study the achievement of both catalysts for the CYSR obtained from two reaction pathways. For this purpose, the reaction between 1 mmol of benzaldehyde (BA) and TMSCN (2 mmol) was selected as a model reaction and performed using TBA-SiW 11 and TBA-PW 11 (2 mol%), separately, without any solvents. Gratifyingly, the desired products were isolated and obtained in 65% and 96%, respectively, after 45 min. Encouraged by these results, different reaction conditions such as temperature, catalyst amount, and solvent were optimized (Table S1). In this context, the selected reaction was accomplished at different temperatures (r.t., 65 °C, and 90 °C) under S.F., which the CYSR led to the highest yield at 65 °C. Moreover, only 55% or 40% of product yield upon performing the reaction at r.t. (Table S1, entries 1 and 2), using TBA-PW 11 or TBA-SiW 11 as catalysts, respectively. Increasing the reaction temperature from 65 °C to 90 °C did not improve the reaction yields (Table S1, entry 14). After that, the effect of various solvents, such as CHCl 3 , MeOH, toluene, and THF, was examined on the CYSR. As evident, S.F. conditions demonstrate the higher activity, leading to a corresponding product yield of 96% or 65% for www.nature.com/scientificreports/ TBA-PW 11 and TBA-SiW 11 , respectively (Table S1, entries 3 and 4), while, using other solvents was not suitable for CYSR due to the production of low-yield products in the range of trace to 78% (Table S1, entries 10-13). For finding the optimum catalyst amount, the CYSR was performed over a different amount of both catalysts from 1 to 3 mol%, and significant development of the product yield was observed from 45 to 96% for TBA-PW 11 or from 35 to 80% for TBA-SiW 11 (Table S1, entries 3, 4 and 6-9), noteworthy, the use of 3 mol% of both catalysts had no effect on improving the CYSR. Therefore, the best catalyst amount was achieved as 2 mol%. Finally, a   Table 1. Representation of important absorption bands (cm −1 ) for TBA-PW 11 and TBA-SiW 11 heterogeneous and nano catalysts.  (Table S1, entry 5). Therefore, the best conditions for promoting CYSR of the BA and TMSCN concern 2 mol% of the TBA-PW 11 as the best catalyst at 65 °C without any solvent. Following, the performance of TBA-PW 11 as the best catalyst was tested towards different substituted ketones and aldehydes ( Table 2). As tabulated, various aldehyde-containing compounds with different electron densities could tolerate these reactions to provide the desired products in high yields. Generally, the aldehydes bearing electron-withdrawing groups, for instance, nitro, bromo, and chloro groups, show the excellent activity with the highest yields, and the potential of the -para position was significant in advancing the reaction to the ortho and meta positions ( Table 2, entries 1-4 and 7-9). In contrast, the aldehyde bearing electron-donating groups (methyl and methoxy groups) exhibit lower yields in longer reaction time ( Table 2, entries 5 and 6). Moreover, ketone compounds with electron-withdrawing groups show moderate to good reactivity and produce the corresponding phenyl-trimethylsilanyloxy-acetonitrile derivatives ( Table 2, entries 10-13), being also above the corresponding yields for the ketones with electron-donor substituents. However, the excellent reactivity of aldehydes compared to ketones is quite apparent. These behaviors are in agreement with the predictable effect of the substituent on the electrophilic character of the carbonyl groups to undergo attack by the -cyano group of TMSCN.

Compound ν as (X-O a ) ν s (X-O a ) ν as (W-O t ) ν as (W-O b ) and ν as (W-O c ) ν (C-H)
Aiming to evaluate the benefits of this study, the catalytic activity of the TBA-PW 11 towards the CYSR of BA with TMSCN was compared with other literature, as shown in Table 3. As tabulated, TBA-PW 11 shows high efficiency, in a shorter time, under solvent-free conditions compared to TBA-SiW 11 and other reported catalysts (Yield of 96%, in S.F. at 65 °C after 45 min, Table 3, entry 6).
Commensurate with the experimental results and previously reported literatures, a possible CYSR mechanism is proposed and illustrated in Fig. 4 8,49 . First, the carbonyl group in BA was activated by the coordinatively central P or Si atoms in catalysts (I) to nucleophilic attack of CN group in TMSCN (II). Finally, with the migration of the silyl group to the oxygen of intermediate (III), a carbon-carbon bond and then cyanohydrin (IV) is formed (Fig. 4). Notably, the products were replaced by BA, and the catalysts were continued to activate the BA in the next catalytic cycle.

Experimental
Chemicals and materials. The chemical compounds were purchased from Merck (Darmstadt, Germany, www. merck milli pore. com) and Sigma-Aldrich (St. Louis, MO, USA, www. sigma aldri ch. com) and used with no crystallization or purification. To conduct CYSR, aromatic aldehydes and ketones, TMSCN, toluene, methanol, chloroform, and THF were used.
Typical method for the CYSR of carbonyl compounds. In a tube, a mixture of a carbonyl compound (1 mmol), TMSCN (2 mmol), and 2 mol% of TBA-PW 11 or TBA-SiW 11 was prepared, and it was put in an oil bath. After that, the mixture was heated at 65 °C without any solvent, for the desired time. Upon completion of CYSR, both mentioned catalysts were separated by filtration, and the mixture's solvent was evaporated. Finally, the pure product was dissolved and achieved in CH 2 Cl 2 .
Catalyst recyclability. Moreover, for examining the heterogeneous nature of the TBA-PW 11 and TBA-SiW 11 , both catalysts separated from the reaction after 20 min and kept the catalyst-free reaction under a similar environment for 25 min more. After removing the TBA-PW 11 and TBA-SiW 11 catalysts from the reaction mixture, no noticeable rise in product yield was detected, which verifies the heterogeneous nature of both catalysts. www.nature.com/scientificreports/ Further, to explore the recyclability of both catalysts, the catalytic activities of the fresh and reused TBA-PW 11 and TBA-SiW 11 were studied and compared. For this purpose, after completing each reaction cycle, catalysts were separated by simple filtration, washed with EtOH, and dried. As exhibited in Fig. S3, TBA-PW 11 and TBA-SiW 11 catalysts could be effectively recycled three times. However, the TBA-SiW 11 catalyst experienced a significant loss in catalytic activity compared to the TBA-PW 11 catalyst. Finally, to check the structural integrity, FTIR analysis of the fresh and recycled TBA-PW 11 and TBA-SiW 11 were recorded. As shown in Fig. 5A, no momentous changes in their patterns were detected. In addition, to elucidate whether the recycling process can result in any change in the catalyst's morphology and catalyst structure, the 1 HNMR spectra and the SEM images of the recycled TBA-PW 11 catalyst were recorded (Fig. 5B,C). These results support that the structure of the TBA-PW 11 underwent several reactions was preserved, but some agglomeration is evident. Characterization data. Spectral data for catalysts. TBA-PW 11

Concluding remarks
In summary, two nano-sized organic-inorganic hybrid systems based on lacunary Keggin TBA-PW 11 and TBA-SiW 11 as heterogeneous catalysts were synthesized and characterized using a suite of analytical techniques. Due to the coexistence of the high negative charge of the above catalysts, they showed an excellent catalytic effect for cyanosilylation of various aldehydes and ketones, giving the corresponding cyanohydrin trimethylsilyl ethers with high yields in a short time. Notably, both catalysts were heterogeneous, but TBA-PW 11 showed higher catalytic activity and recyclability towards the cyanosilylation of aldehydes under S.F. conditions (96%) in comparison  www.nature.com/scientificreports/ with TBA-SiW 11 . Also, further studies are underway in our laboratory to extend the application of these family catalysts to other coupling reactions.

Data availability
All data generated or analysed during this study are included in this published article (and its Supplementary  Information files).